Heart rates and body temperatures are substantially influenced, either directly or indirectly, by both the autonomic and central nervous systems, by the endocrine system, and by metabolism. Heart rate and temperatures, particularly in combinations, can therefore inherently serve to index many and various interactive responses of animals to their environments. These responses include changes in activity, emotions, health, energy allocations, behavioral patterns, and biological rythms. To develop indices based on physiological parameters, such as heart rate and body temperature, to assess animal responses it is often necessary to continuously record the parameters for long periods. The changes that are then observed can be related to the context of the experiment and to the stimuli that gave rise to the responses.
Radiotelemetry has made it possible to monitor physiological parameters in unrestrained subjects for long test periods. Considerable effort has been made to develop biotelemetry systems small enough to be implanted in the most commonly used laboratory animals. U.S. Pat. No. 3,453,546, July 1, 1969, Thomas B. Fryer, for example, reveals an implantable telemeter capable of measuring temperature and pressure.
Difficulties have been encountered in finding suitable electrodes for providing ECG biopotentials in active unrestrained subjects wherein the electrodes are chronically implanted. When conventional ECG electrodes are moved, artifacts are generated which tend to mask the desired biopotentials. If the biopotentials are very weak, it may be impossible to distinguish them from the artifacts. It has been found that when intracardial stimulating electrodes are used for detecting biopotentials, artifact generation is usually more pronounced than it is with standard implantable ECG electrodes.
There is more information available on external ECG electrodes than internal ones. In the past, designers of external electrodes worried about maintaining a high load-to-source impedance ratio to prevent amplifier loading, signal distortion and extraneous 60-Hertz noise. Therefore, they chose external electrodes with large distributed surface areas in order to compensate for the high contact impedance caused by the cornified epithelium of the skin. Of course, as the electrode area was increased, the contact impedance was reduced. Electrolytic pastes and jellies were also employed to reduce the contact impedance. Internal electrodes do not encounter a cornified epithelium and they are rarely bothered by 60-Hertz stray noise. The body fluid that engulfs the electrode when it is implanted in tissue serves an an electrolyte. Investigators such as Geddes and Baker have stated that subcutaneously implanted stainless steel needle electrodes have a low enough impedance to prevent amplifier loading and signal distortion (Med. & Biol. Engng. Vol. 4, pp. 439-450, Pergamon Press, 1966).
The typical approach to the internal electrode artifact problem has been to provide an electrode of a very small surface area which could be securely anchored against the tissue. Electrodes of this type have included small hooked wires, needles and wire loops. Electrodes and the Measurement of Bioelectric Events, L. A. Geddes, John Wiley and Sons, New York 1972; Introduction to Bioelectrodes, C. D. Ferris, Plennum Press, New York 1974; and Medical Instrumentation, Application and Design, J. G. Webster, ed., Houghton Mifflin Co., Boston 1978. A departure from this philosophy is found in U.S. Pat. No. 4,219,027 which discloses a smooth, stainless steel disc ECG electrode.
Body-implantable electrodes for the intracardial stimulation of a heart are revealed in the following U.S. Pat. Nos. 3,664,347; 3,788,329; 3,804,098; 3,911,928 and 4,135,518. These patents all show endocardial electrodes housed in flexible catheters. Intracardial electrodes are used with pacemakers and they are often designed with very small electrode areas to minimize the current flux and the drain on the pacemaker power supply.